Metal material and method for production thereof

ABSTRACT

A metal material comprises a Zn—Al—Sn based alloy (ZAS alloy) and Cu diffused in the alloy, wherein Cu is diffused into the inside of the alloy to a depth from the surface of 0.5 mm or more, the concentration of Cu decreases from the surface of the ZAS alloy towards the inside thereof, and there is present no specific interface between Cu and the ZAS alloy; and a method for producing the metal material involves applying, to the surface of the ZAS alloy, an agent comprising a solvent and, dispersed therein, a material containing Cu such as a Cu powder and a Cu—Mn alloy powder and preferably, dispersed or dissolved therein, a reducing agent capable of reducing an oxide film present on the surface of the ZAS alloy, and heating the ZAS alloy having the agent applied thereon, to thereby diffuse Cu into the alloy.

TECHNICAL FIELD

The present invention relates to a metal material, especially a zincalloy which is excellent in strength, hardness, and heat resistance, anda method of producing the same.

BACKGROUND ART

A variety of surface treatments are applied to metal materials in orderto improve various characteristics such as abrasion resistance,corrosion resistance, and strength. Surface treatments includecarburization, sulfurizing, nitriding, and carbonitriding. In othercases, a coating may be provided by means of, for example, the physicalvapor deposition (PVD) method, the chemical vapor deposition (CVD)method, the plating, and the anodic oxidation.

For example, the means for hardening the surface of a Zn alloy such asZn—Al—Sn alloys is exemplified by a direct electroless nickel platingmethod disclosed in Japanese Patent No. 2832224. In this method, a diecomposed of the Zn alloy is immersed in an electroless nickel platingsolution containing an organic acid nickel salt or the like to form anickel coating on the surface of the die.

According to Japanese Patent No. 2832224, the Zn alloy coated withnickel coating as described above is satisfactory in abrasion resistanceand corrosion resistance.

However, in any one of the methods as described above, the improvementin various characteristics is limited to the surface of the metalmaterial. For example, in the case of the nitriding and thecarburization, the element is diffused only by several tens μm, or about200 μm at the maximum from the surface of the metal material. It isdifficult to improve various characteristics in other regions disposedmore internally than the foregoing.

This inconvenience similarly occurs in the coating formation asrepresented by the invention disclosed in Japanese Patent No. 2832224described above. Further, in this case, the interface exists between thecoating and the metal material. Therefore, when the coefficient ofthermal expansion of the coating is extremely different from that of themetal material, the film may be exfoliated by repeating the heating andthe cooling.

Further, in certain metal materials such as Zn alloys, Al alloys, and Tialloys which have an oxide film formed quickly on the surface, themethod of forming the coating may be limited to the plating, the anodicoxidation, or the like. By such methods, the thickness of the coating issmall. Therefore, various characteristics cannot be sufficientlyimproved.

DISCLOSURE OF THE INVENTION

A principal object of the present invention is to provide a metalmaterial which has sufficient strength, hardness, and heat resistancefrom the surface to the inside of the metal material, and a method ofproducing the same.

According to one aspect of the present invention, there is provided ametal material comprising a diffusion layer containing an elementdiffused in a base material of a metal, wherein the element is diffusedfrom a surface to inside of the base material to a depth of not lessthan 0.5 mm; and

a concentration of the element is gradually decreased from the surfaceto the inside of the base material.

In the present invention, the distance of diffusion of the element isremarkably large as compared with a case in which the treatment such asthe carburization and the nitriding is applied. Accordingly, variouscharacteristics such as the heat resistance, the strength, the hardness,and the corrosion resistance are improved to the deep inside.

The metal material, which serves as the base material, is notspecifically limited. However, preferred examples may include Zn, Znalloy, Al, Al alloy, Mg, Mg alloy, Cu, Cu alloy, Ti, Ti alloy, Fe, andFe alloy.

The Zn alloy is provided with the heat resistance, the strength, and thehardness in a variety of forms. In a preferred aspect, an alloy layer,which is harder than the base layer, is formed at the surface layer. Thealloy layer includes an Fe alloy layer which is formed on the surface,and a diffusion layer which is formed between the Fe alloy layer and thebase layer. A part of copper or manganese, which is contained in thediffusion layer, is diffused to the base layer.

In another preferred aspect, the alloy layer formed at the surface layerincludes a brass diffusion layer containing at least one of iron,nickel, chromium, molybdenum, cobalt, and ceramics.

According to another aspect of the present invention, there is provideda method of producing a metal material comprising a diffusion layerwhich is formed by diffusing an element into a base material of a metaland which has a depth from a surface of said base material of not lessthan 0.5 mm, a concentration of the element being gradually decreasedfrom the surface to the inside of the base material, the methodcomprising:

coating said surface of said base material with a coating agent, thecoating agent including a powder of a substance containing the elementto be diffused, and the powder of the material being dispersed ordissolved in a solvent; and

diffusing the element into the base material by heating the basematerial which is coated with the coating agent. The metal material canbe obtained easily and conveniently by heating after applying the powderwith the solvent.

When the base material is a metal material which readily forms an oxidefilm of, for example, Zn alloy or Al alloy, it is preferable that areducing agent for reducing the oxide film is applied together with thesubstance, for the following reason. The oxide film is reduced todisappear under the action of the reducing agent. Therefore, the elementcan be diffused onto the base material without supplying an extremelylarge amount of thermal energy.

In still another aspect concerning the Zn alloy, a powder of ahydrocarbon compound and at least one metal powder of magnesium,aluminum, or manganese, or at least one alloy powder of magnesium alloy,aluminum alloy, or manganese alloy are dispersed in an organic solventto obtain a powdery dispersing agent. When the surface of the Zn alloyis coated with the powdery dispersing agent, and the Zn alloy isheat-treated thereafter, then the oxide film is removed from the Znalloy.

In still another aspect concerning the Zn alloy, the base material (Znalloy) is processed to have a predetermined shape, and then a firstpowder containing at least one of copper and manganese and a secondpowder of Fe alloy are successively applied to at least a part of thebase material. Subsequently, the portion, to which the first powder andthe second powder have been applied, is heated in an inert atmosphere.Accordingly, it is possible to reliably obtain the Zn alloy which has ahigh strength surface layer and which is excellent in heat resistance.The Zn alloy can be used for a variety of parts such as dies favorably.

Alternatively, after processing the base material (Zn alloy) to have apredetermined shape, at least a part of the base material may be coatedwith a powder containing an essential component of copper or manganeseand containing at least one of iron, nickel, chromium, molybdenum,cobalt, and ceramics. Subsequently, the portion which is coated with thepowder may be heated in an inert atmosphere.

In still another aspect concerning the Zn alloy, at least one of copperand manganese is added as a seeding agent to a molten metal when castingis performed by using the molten metal of Zn or Zn alloy.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates, in cross section, a die of a metalmaterial according to a first embodiment of the present invention;

FIG. 2 schematically illustrates, in cross section, a die of a metalmaterial according to a second embodiment of the present invention;

FIG. 3 is a flow chart illustrating a method of producing the die of themetal material according to the first embodiment;

FIG. 4 illustrates steps of removing an oxide film formed on a surfaceof a ZAS alloy (Zn alloy) as a base material;

FIG. 5 schematically illustrates, in cross section, a die produced by aproduction method according to another embodiment of the presentinvention;

FIG. 6 schematically illustrates a casting apparatus used for the methodof producing the die shown in FIG. 5;

FIG. 7 is a flow chart illustrating the method of producing the dieshown in FIG. 5;

FIG. 8 is a flow chart illustrating a method of producing the die shownin FIG. 2;

FIG. 9 shows steps of producing the die shown in FIG. 2;

FIG. 10 is a schematic front view illustrating an entire test piece;

FIG. 11 illustrates a corrosion test with an aluminum molten metal forthe test pieces as shown in FIG. 10;

FIG. 12 illustrates, in perspective view, a die for which a durabilitytest was carried out;

FIG. 13 illustrates the relationship between the seeding timing and thechange of physical properties;

FIG. 14 illustrates the relationship between the distance from thesurface and the hardness change when the seeding timing is 30 seconds;and

FIG. 15 illustrates a hardness distribution inwardly from a surface of abase material of which an oxide film is removed from the surface.

BEST MODE FOR CARRYING OUT THE INVENTION

The metal material and the method of producing the same according to thepresent invention will be explained in detail below with reference tothe accompanying drawings as exemplified by preferred embodiments.

At first, FIG. 1 schematically illustrates, in cross section, a die 10of a metal material according to a first embodiment of the presentinvention. In the die 10, one or more elements are diffused into a basematerial 12, and a diffusion layer 14 is formed thereby. Preferred metalmaterials of the base material 12 may include Zn alloy, Al alloy, Mgalloy, Cu alloy, Ti alloy, and Fe alloy which are widely used as alloysof practical use. However, there is no special limitation thereto.

The most diffused element in the base material 12 of the metal materialas described above arrives at a depth from the surface of the basematerial 12 of at least 0.5 mm (500 μm). The depth may be 2 cm (2000 μm)at the maximum. This value is remarkably large as compared with severaltens of μm or about 200 μm of the diffusion distance of the elementachieved, for example, by the conventional nitriding and thecarburization methods. That is, the diffusion distance of the elementachieved in the present invention has the remarkably large value ascompared with the diffusion distance of the element by the conventionalsurface treatment method.

The type of the element to be diffused depends on the type of the metalmaterial to serve as the base material 12. Selected elements are capableof improving various characteristics of the metal material. For example,when the base material 12 is composed of Zn alloy, it is possible toselect at least any one of Cu and Mn. In this case, the diffusion layer14 may further contain at least one of Fe, Ni, Cr, Mo, Co, and ceramics.

When the base material 12 is composed of Fe alloy, Cr may be diffused.When the base material 12 is composed of Ti alloy, at least any one ofAl, Cr, Ni, and N may be diffused. When the base material 12 is composedof Cu alloy, Ni may be diffused.

The form of the element, which exists after the diffusion into the die10, is not specifically limited. That is, the element may form an alloytogether with the metal material of the base material 12. Alternatively,the element may form a compound together with any impurity contained inthe metal material. Further alternatively, the element may form a solidsolution singly.

As described later on, the element is diffused from the surface of thebase material 12. Therefore, the concentration of the element in thediffusion layer 14 is the highest at the surface, and the concentrationis gradually decreased inwardly. In FIG. 1, a boundary line is depictedbetween the diffusion layer 14 and the base material 12 for illustrationpurpose. However, actually, distinct interface does not exist betweenthe diffusion layer 14 and the base material 12.

In the die 10 as described above, various characteristics of the basematerial 12 are improved over the region where the diffusion layer 14exists, in other words, to the depth to which the element has beendiffused. For example, when Cu is diffused into the base material 12 ofZn—Al, Zn—Sn, or Zn—Al—Sn alloy (so-called ZAS alloy) as the Zn alloy,Cu is bonded to Zn to form Cu—Zn alloy (brass). Both of the strength andthe hardness of the brass are twice or more, compared with the strengthand the hardness of Zn. Further, the brass is excellent in corrosionresistance. Furthermore, the melting point of the brass is twice ormore, compared with the melting point of Zn. Therefore, the meltingpoint is raised owing to the production of the brass. As a result, theheat resistance is improved. Consequently, the obtained diffusion layer14 is excellent in various characteristics such as the strength, thehardness, the corrosion resistance, and the heat resistance.

In the die 10, no distinct interface exists between the diffusion layer14 and the base material 12. Accordingly, the occurrence of stressconcentration is avoided. Therefore, it is also possible to suppress theincrease in brittleness or fragility which would be otherwise increasedwhen the element is diffused.

When Fe, Ni, Cr, Mo, or Co is further contained in the diffusion layer14, each of the elements improves the strength, the hardness, and thecorrosion resistance of the Zn alloy. When some ceramics are added, thestrength and the hardness are improved, and the abrasion resistance isimproved. Therefore, the diffusion layer 14 can be reliably obtained,which is excellent in hardness, strength, corrosion resistance, or thelike as compared with the base material 12.

Further, Cu functions as an excellent binder for Fe, Ni, Cr, Mo, Co, orceramics. Accordingly, it is possible to provide the diffusion layer 14which has the high corrosion resistance and the abrasion resistance.

FIG. 2 schematically illustrates, in cross section, a die 20 of a metalmaterial according to a second embodiment. The die 20 has a basematerial 22 which is composed of ZAS alloy, and an alloy layer 24 whichis harder than the base material 22. In particular, the alloy layer 24is composed of an Fe alloy layer 26 formed near the surface, and adiffusion layer 28 which is formed between the Fe alloy layer 26 and thebase material 22.

The Fe alloy layer 26 is formed so that the thickness H1 is within arange of 0.5 mm to 1.5 mm from the surface. On the other hand, thediffusion layer 28 contains at least any one of Cu and Mn. A brass layeris provided near the Fe alloy layer 26. The brass layer is selected fromZn—Cu, Zn—Mn—Cu, Zn—Al—Cu, Zn—Al—Cu—Mn, Zn—Sn—Cu, Zn—Sn—Cu—Mn,Zn—Sn—Al—Cu, and Zn—Sn—Al—Mn—Cu, for example. An Mn alloy layer isprovided in the brass layer. The Mn alloy layer is selected from Zn—Mn,Zn—Sn—Mn, Zn—Al—Mn, and Zn—Al—Sn—Mn, for example. The diffusion layer 28is designed so that the thickness H2 starting from the inner boundaryline of the Fe alloy layer 26, is within a range of 0.5 mm to 30 mm.

In this arrangement, an alloy layer 24 provided with the Fe alloy layer26 and the diffusion layer 28 is formed on the surface of the basematerial 22. In other words, the Fe alloy layer 26 is provided as thesurface layer of the die 20. Accordingly, the melting point, thestrength, the hardness, and the heat resistance are remarkably improvedon the surface of the die 20 as compared with the Zn alloy as the basematerial 22. As a result, it is possible to reliably improve variouscharacteristics such as the abrasion resistance, the heat resistance,and the shock resistance.

Further, the diffusion layer 28 exists as the intermediate layer betweenthe Fe alloy layer 26 and the base material 22. The diffusion layer 28contains the brass layer including Cu and Zn. Therefore, the meltingpoint, the strength, the hardness, and the heat resistance of thediffusion layer 28 are improved as compared with the Zn alloy as thebase material 22.

In this arrangement, when the component ratio in the diffusion layer 28is gradually changed, then no interface exists, and the exfoliation andthe stress concentration, which would be otherwise caused by thedifference in thermal expansion, can be effectively avoided.Accordingly, it is possible to use the die 20 favorably for a longperiod of time, so that the die 20 is extremely economical.

The Fe alloy layer 26 is formed inwardly from the surface within a rangesuch that the thickness H1 is 0.5 mm to 1.5 mm. Further, the diffusionlayer 28 is formed inwardly from the Fe alloy layer 26 within a rangesuch that the thickness H2 is 0.5 mm to 30 mm. Accordingly, it ispossible to reliably avoid the occurrence of, for example, the breakageand the crack especially during the use of the thermal cycle, ascompared with a case in which a coating treatment such as the plating,CVD, PVD, and the anodic oxidation is applied to the base material 22.

If the thickness H1 of the Fe alloy layer 26 is less than 0.5 mm, thephysical properties are not improved. On the other hand, if thethickness H1 exceeds 1.5 mm, the processability is deteriorated. If thethickness H2 of the diffusion layer 28 is less than 0.5 mm, the physicalproperties are not improved. On the other hand, if the thickness H2exceeds 30 mm, then the diffusion requires a long period of time, and itis impossible to realize efficient production.

Next, the method of producing the die 10, 20 will be explained, asexemplified by a case in which Cu is diffused in the ZAS alloy.

FIG. 3 is a flow chart of the production method to obtain the die 10.The production method comprises a first step S1 of coating a surface ofa base material with a substance containing an element to be diffused,and a second step S2 of diffusing the element into the base material byheating.

At first, as shown in FIG. 4, a machining treatment is applied to thebase material 12 (ZAS alloy) by using a processing machine 30 to form asemimanufactured product having a shape corresponding to the die 10.

On the other hand, a processed surface S of the semimanufactured productis coated with a coating agent P in the first step S1. As for thesolvent of the coating agent P, it is preferable to select an organicsolvent which is readily volatile, such as acetone and alcohol. Thesubstance containing Cu is dispersed in the solvent.

The substance containing Cu is exemplified, for example, by Cu powderand Cu—Mn alloy powder. In particular, it is preferable to select Cu—Mnalloy powder in view of the fact that this substance has a relativelylow melting point, for the following reason. When this substance isselected, Cu can be dispersed at a lower temperature, in other words,with a smaller amount of thermal energy. As for the Cu—Mn alloy, forexample, it is possible to use one in which the composition ratiobetween Cu and Mn is 6:4 in molar ratio.

Usually, an oxide film is formed on the surface of the ZAS alloy. Inorder to diffuse Cu in this state, it is necessary to supply anextremely large amount of thermal energy so that Cu is successfullypasses through the oxide film. In order to avoid this inconvenience, itis preferable that a reducing agent for reducing the oxide film is mixedwith the coating agent P.

Specifically, a substance, which acts as the reducing agent on the oxidefilm and which does not react with the ZAS alloy, is dispersed ordissolved in the solvent. Preferred examples of the reducing agent mayinclude each of resins of nitrocellulose, polyvinyl alcohol, polyvinyl,acrylic, melamine, styrene, and phenol. However, there is no speciallimitation thereto. The concentration of the reducing agent may be about5%.

It is preferable that a powder of at least any one of Mg, Mg alloy, Al,Al alloy, Mn, and Mn alloy is further added to the coating agent P, forthe following reason. Each of Mg, Al, and Mn is promptly bonded to O ascompared with Zn. Therefore, it is possible to avoid the oxidation ofthe Zn alloy again after reducing and removing the oxide film.

It is preferable that at least one of the metals described above ismixed with a metal with which oxygen is diffused more promptly. Such ametal is at least one of Ni, Sn, and Cu. It is desirable that the metalis mixed or alloyed to be formed into a powdery form. As for such ametal, the diffusion of oxygen in the metal is caused extremely quicklywithin a temperature range of 250° to 350° C. It is possible to greatlyimprove the alloy formation efficiency and the alloy formation speed.When the alloy formation is advanced, the melting point is raised aswell, the heating at temperatures of not less than 350° C. is advanced,and the alloy formation is further facilitated.

When Fe, Ni, Cr, Mo, Co, or ceramics is further contained in thediffusion layer 14 (see FIG. 1), a powder of each of them may be furtheradded to the coating agent P.

With the coating agent P, which has been prepared by mixing thematerials as described above, the processed surface S is coated with acoating machine 32. After that, in the second step S2, the heat isapplied to the ZAS alloy to which the coating agent P has been applied.That is, the semimanufactured product, which is coated with the coatingagent, is arranged in a heating apparatus 34. The semimanufacturedproduct is heated by using a heating source 35 such as a burner or aheater in an inert atmosphere such as a nitrogen (N₂) gas atmosphere.

In this procedure, the semimanufactured product may be heated in a statein which a temperature gradient is provided in the second step S2. Thatis, a plate member for avoiding any excessive heating abuts against oneend surface of the semimanufactured product. In this state, thesemimanufactured product may be heated from another end surface oppositeto the end surface where the plate member abuts. The heat is absorbed bythe plate member as described later on. Therefore, Cu can be diffusedwithout melting the semimanufactured product.

During the process in which the temperature is raised, the reducingagent begins to be decomposed at about 250° C., and carbon and hydrogenare produced. The oxide film formed on the surface of thesemimanufactured product disappears as a result of the reduction by thecarbon and hydrogen. Accordingly, it is unnecessary for Cu to passthrough the oxide film. Therefore, it is possible to shorten the timerequired for the diffusion, and it is possible to reduce the thermalenergy required.

Further, in this procedure, it is unnecessary to use any specialexclusive equipment unlike the procedure of reduction with hydrogen. Theoxide film formed on the base material 12 can be reliably removed bymeans of the simple construction and the steps.

When at least one metal powder of Mg, Al, and Mn, or an alloy powder ofeach of them is added to the coating agent P, the powder promptly reactswith oxygen as compared with Zn which is the major component of the basematerial 12 (ZAS alloy). Therefore, it is possible to avoid theoxidation of Zn again, and the diffusion preformed thereafter issmoothly advanced.

When the temperature is continuously raised, Cu begins to diffuse intothe semimanufactured product (ZAS alloy). When the Cu—Mn alloy powder isapplied, the diffusion is started at a low temperature as compared witha case in which the Cu powder is used. As a result of the diffusion, thediffusion layer 14 is formed, and the die 10 is finally obtained.

The diffused Cu is ultimately bonded, for example, to Zn of the ZASalloy to form the Cu—Zn alloy. As a result, the melting point of the ZASalloy (die 10) is raised. Therefore, the die 10 is not melted.

When the plate member abuts against one end surface, the heat suppliedto the die 10 is transmitted to the plate member, and then the heat isconsumed by raising the temperature of the plate member. In other words,the heat is absorbed by the plate member. Accordingly, it is possible toefficiently diffuse Cu without melting the die 10.

The degree of the diffusion of the element also depends on the shape ofthe base material to be heated. For example, when the ZAS alloy is acube of 100 mm×100 mm×100 mm, it is possible to diffuse Cu to a depth ofabout 1.5 mm from the surface of the cube. The concentration of Cu isgradually decreased. No distinct interface appears between the ZAS alloyand the end of the diffusion of Cu as well.

The hardness and the strength are remarkably improved in the die 10(Cu-diffused ZAS alloy) obtained as described above, as compared with adie of only the ZAS alloy (only the base material 12) in which Cu is notdiffused. Specifically, the Vickers hardness (Hv) of the surface of thebase material 12 is about 120, and the tensile strength thereof is about200 MPa. On the other hand, in the case of the Cu-diffused ZAS alloy(die 10), Hv of the surface and the tensile strength are about 250 andabout 450 Mpa, respectively, both of which are about twice the above.

Mn can also be diffused in the ZAS alloy in the same manner as describedabove.

When Cr is diffused in an Fe alloy represented, for example, by S45C(JIS Standard), the following procedure may be adopted. An acrylic resinmonomer is dissolved in acetone so that the concentration is 0.5%. Amixed powder, in which respective powders of Cr, Mo, Ni, C, and BN aremixed in a ratio (weight ratio) of 2:3:4:0.5:0.5, is dispersed thereinto prepare a coating agent.

S45C is coated with the coating agent (first step S1), and then a heatis applied in an electric furnace (second step S2). S45C is a highmelting point substance, and it is hardly melted. Therefore, the heatingtemperature can be about 1200° C. The temperature may be retained forabout 1 hour.

When the heat is applied in the electric furnace, it is preferable touse an inert atmosphere of, for example, nitrogen or argon. Accordingly,it is possible to avoid the oxidation of the surface of S45C.

In this procedure, it is especially unnecessary that the plate memberabuts against one end surface of the base material in order to avoid anyexcessive heating, for the following reason. S45C is the high meltingpoint substance as described above, and hence it is especiallyunnecessary to absorb the heat in order to prevent S45C from beingmelted during the heat treatment at the high temperature. For the reasondescribed above, it is preferable that the second step S2 is carried outin the inert atmosphere.

After the completion of the second step S2, chromium carbonitride isproduced on the surface of S45C, and Cr is diffused in S45C. In thiscase, Cr is diffused to a depth of 1.8 mm from the surface. Theconcentration thereof is gradually decreased, and no interface appearsbetween Cr and S45C.

The surface of the Cr-diffused S45C obtained as described above has Hvof 650 which is an extremely high value.

The volume change before and after the second step S2 is remarkablysuppressed to be 0.216%, because of the production of chromiumcarbonitride on the surface. The strain energy accumulated in thisprocess is approximately calculated to be about 102 MPam. This indicatesthat the large strain energy can be accumulated in the hardeningoperation and the tempering operation.

Next, an explanation will be made about an example of diffusion of Al,Cr, Ni, and N in Ti-6Al-4V alloy.

A coating agent, with which the surface of the Ti-6Al-4V alloy iscoated, is prepared in the first step S1 in the same manner as describedabove. In this procedure, a powder of metal element which readily formsan intermetallic compound together with Ti in the Ti alloy, for example,a mixed powder of Al, Cr, and Ni powders may be dispersed, in acetone oralcohol.

An oxide film also exists on the surface of the Ti-6Al-4V alloy.Accordingly, also in this case, it is preferable that the coating agentis mixed with a reducing agent capable of reducing the oxide film, forexample, a powdery carbon material.

Further, a BN powder may also be mixed with the coating agent, sinceTiB₂ which is a boride of Ti is obtained. The hardness of the alloy canbe improved by dispersing TiB₂ in the Ti-6Al-4V alloy as the basematerial.

In view of the fact described above, it is preferable in this procedureto use the coating agent containing the powder mixed with the Al powder,the Cr powder, the Ni powder, the C powder, and the BN powder, forexample, in a ratio (weight ratio) of 30:10:50:5:5.

The surface of the Ti-6Al-4V is coated with the coating agent to have athickness of about 0.5 mm by means of the known coating technique suchas the brush coating method. After that, the heat is applied in thesecond step S2 in the same manner as described above. The heat treatmentmay be carried out, for example, in a heat treatment furnace which is anitrogen atmosphere.

In this procedure, the temperature is raised at a speed of 10° C./minutewhile allowing nitrogen to flow so that the pressure is 10 Pa, and thetemperature is retained for 30 minutes at 250° C., 450° C., and 650° C.,respectively. After that, the pressure is changed to 0.3 MPa, thetemperature is raised up to 777° C. at 5° C./minute, and the temperatureis retained for 1 hour so that the heat may be applied. Accordingly, theoxide film on the surface of the Ti-6Al-4V alloy is reduced. The metalelements contained in the coating agent and N originating from nitrogenin the atmosphere can be reliably diffused into the alloy.

The diffused element, for example, Al is ultimately bonded, for example,to Ti of the Ti-6Al-4V alloy to form Al—Ti alloy. Further, for example,chromium nitride and titanium nitride are produced in accordance withthe nitriding of Cr and Ti remaining on the surface. Furthermore, TiB₂is produced as a result of bonding between Ti and B. As a result, adiffusion layer of ceramics or alloy is formed in the Ti-6Al-4V alloyafter the heat treatment.

When the Ti-6Al-4V alloy has a columnar shape having a diameter of 15 mmand a length of 100 mm, the alloy of Ti and Al, Cr or Ni can be producedto a depth of about 2.3 mm from the surface of the column by the heattreatment as described above. Further, it is possible to producechromium nitride, titanium nitride, and TiB₂. The concentration of thealloy or the ceramics is gradually decreased. No distinct interfaceappears between the Ti-6Al-4V alloy and the end of the diffusion of thealloy or the ceramics as well.

Also in this case, various characteristics are remarkably improved ascompared with the Ti-6Al-4V alloy before the diffusion. Specifically, Hvof the surface of the Ti-6Al-4V alloy before the diffusion is about 300.On the other hand, Hv of the Ti-6Al-4V alloy after the diffusion is1200.

As shown in FIG. 5, the diffusion layer 14 can also be formed over theentire surface of a die 36. In this case, the product can also bemanufactured by means of the casting with a casting apparatus 40 asshown in FIGS. 6 and 7. The casting apparatus 40, which is schematicallyillustrated in FIG. 6, comprises a molten metal-holding furnace 42 forholding the molten metal L of melted metal of ZAS alloy, a moltenmetal-ladling mechanism 44 for ladling the molten metal L in apredetermined amount (amount of one shot) from the inside of the moltenmetal-holding furnace 42, a seeding agent-adding mechanism 48 for addinga seeding agent SA to the molten metal L ladled by a ladle 46 of themolten metal-ladling mechanism 44, and a mold 50 for molding the moltenmetal L added with the seeding agent SA to have a shape of the die 36.

The seeding agent SA contains at least any one of Cu and Mn, preferablyboth of them. Cu and Mn are pulverized into powdery forms havingparticle sizes of 10 μm to 50 μm, more preferably 10 μm to 20 μmrespectively. If the particle size is less than 10 μm, the alloyformation is excessively advanced, and the diffusion tends to beexcessively advanced. Therefore, the effect to improve variouscharacteristics is inferior. On the other hand, if the particle sizeexceeds 50 μm, the die 36 may suffer from the roughness and the defectof the cast texture.

It is preferable that the seeding amount of Cu is 1% by weight to 18% byweight with respect to the entire ZAS alloy. If the seeding amount isless than 1% by weight, then the diffusion tends to be excessivelyadvanced, and hence the effect to improve various characteristics isinferior. On the other hand, if the seeding amount exceeds 18% byweight, then the molten metal L is quickly cooled, and the quality ofthe obtained cast product may be lowered. More preferably the seedingamount of Cu is 3% by weight to 7% by weight. Within this range, thealloy formation occurs in the vicinity of the surface of the castproduct (die 36) to a depth of several mm to several tens mm. No crystalgrain of zinc or Zn—Al—Sn alloy is observed, which is satisfactory.

On the other hand, the seeding amount of Mn is set to be 3% by weight to30% by weight of the seeding agent SA. If the seeding amount is lessthan 3% by weight, no sufficient effect is obtained. If the seedingamount exceeds 30% by weight, unreacted matters are consequentlyaggregated. The physical properties of the alloy layer 14 may bedeteriorated to cause some defect in the die.

An explanation will be made below with reference to a flow chart shownin FIG. 7 about a method of producing the die 36 constructed asdescribed above.

At first, as shown in FIG. 6, the molten metal L of melted metal of theZAS alloy is held in the molten metal-holding furnace 42 (step S10).When the molten metal-ladling mechanism 44 is operated, the ladle 46,which is inserted into the molten metal-holding furnace 42, is inclined.Accordingly, the molten metal L in an amount of one shot is ladled bythe ladle 46 (step S20).

The ladle 46, with which the molten metal L has been ladled, is moved tothe position for the addition by the seeding agent-adding mechanism 48.The seeding agent SA in the predetermined amount is supplied from theseeding agent-adding mechanism 48 to the molten metal L contained in theladle 46 (step S30). The molten metal-ladling mechanism 44 starts theporing of the molten metal into a pouring port 52 of the mold 50 in 10to 30 seconds after performing the addition of the seeding agent SA(step S40). Accordingly, an unillustrated cavity in the mold 50 isfilled with the molten metal L added with the seeding agent SA.

After that, a predetermined cooling treatment is applied. Accordingly,the die 36 is obtained as the cast formed product (step S50).

As described above, the seeding agent SA, which contains Cu or Mn, isadded to the molten metal L. Therefore, the diffusion layer 14 of brasssuch as Zn—Cu, Zn—Mn—Cu, Zn—Al—Cu, Zn—Al—Cu—Mn, Zn—Sn—Cu, Zn—Sn—Cu—Mn,Zn—Sn—Al—Cu, and Zn—Sn—Al—Mn—Cu is formed as the surface layer of themanufactured die 36.

In this case, it is possible to manufacture the die 36 with ease bymeans of the casting. Further, it is possible to lower the meltingtemperature as compared with a case in which the casting is performed byusing a material in which copper, manganese, or the like is previouslymixed with the ZAS alloy. It is possible to reduce the amount of energyconsumption.

The molten metal L is poured into the mold 50 in 10 to 30 seconds afterthe addition of the seeding agent SA. Accordingly, the seeding agent SAis sufficiently diffused in the molten metal L. As a result, the alloylayer 14 is formed in the die 36 within a range of about several mm to25 mm in the direction directed inwardly from the surface. If thepouring of the molten metal L is performed at a timing less than 10seconds after the seeding, the seeding agent SA (copper and/ormanganese) is not diffused sufficiently in the molten metal L.Therefore, it is impossible to obtain any necessary hardness. On theother hand, if the timing exceeds 30 seconds after the seeding, crystalgrains are grown, resulting in the decrease in hardness.

In the case of the above, the ladle 46, with which the molten metal Lhas been ladled, is moved to the position of the addition by the seedingagent-adding mechanism 48, and the predetermined amount of the seedingagent SA is supplied from the seeding agent-adding mechanism 48 to themolten metal L contained in the ladle 46. However, the seeding agent SAmay be directly supplied to a mold passage which is communicated with amolten metal port or a molten metal passage provided for the mold 50.

Next, an explanation will be made about a method of producing the die 20with reference to a flow chart shown in FIG. 8 and a step diagram shownin FIG. 9.

At first, as shown in FIG. 9, a base material 22 made of ZAS alloy isprepared. A machining treatment is applied to the base material 22 byusing a processing machine 30 (step S100). Accordingly, asemimanufactured product is formed with a processed surface Scorresponding to a cavity and the semimanufactured product correspondsto the shape of the die 20.

Subsequently, the processed surface S is coated with a first paste P1 byusing a first coating means 38 a (step S200). The first paste P1 ismixed with at least one of Cu and Mn, which is prepared, for example, bydiffusing Cu and Mn in a ratio of 4:6 to 6:4 in an organic solvent. Areducing agent and/or an oxygen-capturing agent may be contained in thefirst paste P1 as described above.

Subsequently, the first paste P1 is coated with a second paste P2 byusing a second coating means 38 b (step S300). The second paste P2 isprepared by diffusing an alloy containing a major component of Fe, Ni,Cr, Mo, or Co in an organic solvent.

Subsequently, the semimanufactured product, which is coated with thefirst paste P1 and the second paste P2, is arranged in a heatingapparatus 34 in the same manner as described above. The die 20 is heatedby using a heating source 35 such as a burner or a heater in an inertatmosphere, for example, in a nitrogen (N₂) gas atmosphere (step S400).Accordingly, the die 20, which has the alloy layer 24 of the Fe alloylayer 26 and the diffusion layer 28, is obtained. A finishing treatmentsuch as a surface-polishing treatment is applied to the die 20 (stepS500).

EXAMPLE 1

Test pieces 60 each having a stepped rod shape as shown in FIG. 10 weremanufactured by using a base material 12 made of ZAS alloy.

Subsequently, powders A, B, C, D, E, and F having compositions (% byweight) shown in Table 1 were prepared, and the powders were dispersedin xylene respectively to prepare pasty coating agents. The surfaces ofthe test pieces 60 were coated with the coating agents A, B, C, D, E,and F to have thicknesses of about 0.3 mm, and then dried.

TABLE 1 ALLOY SUR- POR- FACE TION HARD- HARD- COMPO- CER- NESS NESSSITION Cu Ni Cr Co Mo Fe Mn AMICS (Hv) (Hv) A 50 50 320 180 B 40 10 50340 170 C 45 10 1 44 340 190 D 40  5 1 10 44 350 180 E 40  5 1 13 2 39370 180 F 40  5 2 5 40 8 426 180

Further, the respective dried test pieces 60 were heated for 60 minutesat 350° C. under the flow of nitrogen gas. After the heat treatment, therespective test pieces 60 were cut in central cross sections to confirmthe thicknesses of the reacted portions under a metallurgical microscopeand measure the surface hardness (Hv) and the alloy portion hardness(Hv) at the position inwardly from the outermost surface by 5 mm.Obtained results are also shown in Table 1.

On the other hand, a corrosion test with aluminum molten metal wascarried out for the respective test pieces 60 prepared separately andanother test piece 60 which was not coated with the pasty coating agent.Specifically, the respective test pieces 60 were immersed in thealuminum molten metal (corresponding to ADC12) heated to about 700° C.,for 30 minutes, 60 minutes, and 90 minutes respectively. After that, thetest pieces 60 were taken out of the aluminum molten metal, and theywere cut in central cross sections. The change in shape was confirmed,and the corrosion situation was detected.

FIG. 11 shows representative corrosion situations. In the case of thetest piece 60 which was not coated with the pasty coating agent, thetest piece 60 was melted to a great extent, and the original shapethereof was not maintained. On the contrary, in the case of the testpieces 60 to which Powders A to F were applied, it was confirmed thatthe corrosion resistance was greatly improved.

The degree of the melting out was decreased in an order of Powder A,Powder B, Powder C, Powder D, Powder E, and Powder F. Further, thedegree of the melting out was decreased with respect to the immersiontime, and the melting out speed was greatly decreased.

EXAMPLE 2

A die 62 as shown in FIG. 12 is made of Zn—Al—Sn alloy. In the case ofthe die 62 of this type, the cracks were observed after several thousandshots. For example, the cracks began to appear at 1000 shots at thecorner portions. The cracks began to appear at 2000 to 4000 shots at therespective joining surfaces of the die. The cracks were enlarged as thenumber of shots was increased.

In view of the above, a surface treatment was applied to the die 62 byusing Powder A described in Example 1. That is, the die 62 was coatedwith the pasty coating agent to provide a thickness of 1.5 mm, and thena heat treatment was applied at 500° C. for 30 minutes while allowingnitrogen gas to flow. Subsequently, the finishing processing wasperformed, and then the surface hardness was detected. As a result, thesurface hardness was about Hv 200, and the depth of the diffusion layerwas 5 mm.

The number of shots, at which the cracks began to appear in the die 62,was increased from 1000 shots to 18000 shots at the corner portions, andit was greatly increased from 2000 shots to 35000 shots, from 3000 shotsto 45000 shots, and from 4000 shots to 80000 shots at the respectivejoining surfaces of the die respectively.

EXAMPLE 3

ZAS alloy was melted at 600° C. to prepare molten metal L. A treatmentsuch as degassing was applied to the molten metal L, and then the moltenmetal was poured into a mold 50 at 550° C.

At first, in order to investigate the seeding timing, the time was setduring the periods in which the material was in the state of the moltenmetal, the material was contained in the ladle, and the material waspoured, so that the uniformity of the dispersion was observedrespectively and the effect was examined (see FIG. 13). The seedingtiming was defined as the period of time (second(s)) between theaddition of the seeding agent SA and the contact of the molten metal Lwith the pouring port 52 of the mold 50.

The seeding agent SA was a powdery mixture of copper and manganesehaving particle sizes of 10 μm to 20 μm respectively. The amount ofaddition was 5% of the cast matter of the cast product 10.

Respective samples, which were cast by the casting apparatus 40, werecut in central cross sections, and a polishing treatment and amirror-finish treatment were applied to the cross sections. After that,an alkaline corrosion treatment was applied to the surface to observethe change of the crystalline microstructure for the respective samples.The HV hardness was measured at the portion inwardly from the surface by2 mm. Results are shown in FIG. 13.

On the other hand, when a sample was cast without seeding, then thecrystals were in a form of dendrite, and the particle was in a form oftear. The long diameter was 600 μm to 800 μm, and the short diameter was150 μm to 200 μm. The hardness was HV 110 to 120.

As clearly understood from FIG. 13, the change of the crystallinemicrostructure was clearly confirmed visually depending on the seedingtiming. Further, the difference appeared in the crystal size and thehardness. As for the microstructural change, the layer itself wasincreased as the seeding timing was prolonged. However, the seedingagent SA was diffused, and there was not any contribution to theimprovement in hardness and crystal size so much. On the other hand,when the seeding timing was short before starting to cast, i.e., 1second or 5 seconds, then the seeding agent SA was not diffusedsufficiently, and there was no improvement in hardness as well.

Although copper and manganese were simultaneously seeded, the diffusionwidths thereof were different from each other. Manganese was permeatedinwardly by a twice or three times distance as compared with copper.Even when the seeding timing was 30 seconds, the alloy formation portionwas clearly confirmed in a range about 27 to 30 mm. FIG. 14 shows thechange of the hardness in this case.

According to the fact described above, the seeding timing was mostpreferably between 10 seconds and 30 seconds in which the fine crystalformation was improved to be 1/20 as compared with the case in which theseeding was not performed, and the hardness was improved about twice. Inthis experiment, the samples having the seeding timing of 10 seconds and30 seconds were used as tensile test samples each of which was cut outbased on the crystal change portion disposed in the vicinity of thesurface to perform the measurement. As a result, the strengths were 480MPa and 420 MPa which were greatly improved respectively, while thestrength without seeding was 230 MPa.

EXAMPLE 4

A base material 22 made of ZAS alloy was prepared. The surface of thebase material 22 was processed to form a processed surface Scorresponding to a cavity. Further, the processed surface S was cleanedby removing any oil film therefrom.

After removing the oxide film from the processed surface S, theprocessed surface S was coated with a first paste, which containedacrylic resin, cellulose nitrate, and Cu—Mn powder (composition ratio:5:2), so that the thickness was 1.5 mm. Further, the first paste wascoated with a second paste, which was prepared by dispersing Cu—Mn—Fe—Alpowder (composition ratio: 20:15:64:1) and acrylic resin in an organicsolvent, so that the thickness was 2 mm.

Subsequently, the processed surface S of the base material 22, which wascoated with the first and second pastes as described above, was heatedfor 20 minutes with a burner using propane and oxygen. Accordingly, theapplied metals were diffused into the base material 22.

After that, the base material 22 was machined to manufacture a test diehaving a size of 300 mm×300 mm×80 mm and a maximum depth of the cavityof 30 mm. The thickness of the applied film was decreased to 0.9 mmthrough 1.1 mm after the heating.

In this procedure, the outmost surface of the powdery matter wasoxidized by the heating with the heater. However, the thickness of theoxidized region was not more than about 0.2 mm. In the region deeperthan the above, the metallic luster was observed when the oxidized layerwas removed.

An etching treatment was applied for 45 seconds by using 10% NaOH. Theinternal microstructure was observed from the processed surface S of thebase material 22. In this procedure, the thickness of the brass layerwas 7 mm to 9 mm. The diffusion layer deeper than the above was changedto have a position inwardly by about 27 mm from the surface. The changewas clearly observed visually because of the change of the crystals fromthe dendrite, for example, into the cubic crystals and the tesseralcrystals.

On the other hand, in the X-ray observation, the surface of theprocessed surface S was in a metallic luster region in which Fe was 94%and Cu was 5%. In a region disposed inwardly from the surface by 1 mm,Cu was 50% and Zn was 50%. In a region disposed further inwardly by 5mm, Cu was 25%, Mn was 14%, and Zn was 50%.

In a region disposed inwardly from the surface by 10 mm, Cu was 8%, Mnwas 10%, and Zn was 76%. In a region disposed inwardly by 20 mm, Cu was4%, Mn was 5%, and Zn was 82%. In a region disposed further inwardly by30 mm, the composition of the Zn—Al—Sn alloy was observed.

For the purpose of comparison, the heat resistance test and the shockresistance test were performed by using the test die described above anda machined die with no diffusing treatment (hereinafter referred to as“comparative die”). Specifically, the cavity portion was arranged in afurnace heated to 200° C., and it was held for 10 minutes, then plungedinto water having a temperature of 20° C. This procedure was repeatedlyperformed to observe the occurrence of any crack. As a result, in thecase of the comparative die, the crack appeared at the cavity cornerportion of the die at 18 cycles, and the damage was clearly observed at28 cycles.

In contrast, in the case of the test die, no crack was observed at thecorner portion even at 320 cycles, but minute cracks appeared at 374cycles. That is, heat check resistance was remarkably improved in thetest die to which the diffusing treatment was applied, as compared withthe comparative die to which the diffusing treatment was not applied.

EXAMPLE 5

A surface of a base material 10 made of ZAS alloy was processed to forma processed surface S. The processed surface S was finished to have asurface roughness of 1.6 S to 3.2 S, and then a degreasing treatment wasapplied thereto.

A coating agent was prepared by dispersing an Mn—Cu alloy powder (Mn:Cuwas 40:60) having a grain size of not more than 5 μm in an amount of 25%in a solution containing 5% nitrocellulose, 80% acetone, 10% ethanol,and 5% ethylcellosolve.

Subsequently, the entire processed surface S was coated with the coatingagent so that the thickness was 1.0 mm, then left one day at roomtemperature to dry. After that, the processed surface S of the basematerial 10 was subjected to the temperature raising at a speed of 10°C./minute in a nitrogen atmosphere, and then retained at 250° C. for 30minutes. Further, the temperature was raised over 1 hour to 340° to 350°C., and then cooled in the furnace. The cooled base material 10 was cutat the central portion, and a mirror-finish treatment was appliedthereto. After that, the microstructure was observed and the hardnesswas measured.

The thickness of the coating agent on the processed surface S wasdecreased to about 0.3 mm. The metal density upon the application was 40to 50%, and the structure was densified during the heating. However, thethickness was thinner than an assumed thickness. Therefore, it isappreciated that the metal components were permeated and diffused intothe base material 10.

The surface layer ranging from the surface of the base material 10 to aposition inwardly about 1.5 mm was discolored into yellow or gold, inwhich a brass layer was certainly formed. In the surface layer, thecrystals were changed from the dendrite, for example, into the cubiccrystals and the tesseral crystals. The crystal grains were decreasedfrom the size of 1.0 mm to 1.5 mm to the size of about 30 μm to 40 μm.

A layer, which is disposed under the surface layer, was clearlydifferent from the crystalline microstructure of the ZAS material. Thatis, although the dendrite was partially present, the portion forsurrounding the same was changed. The portion was revealed to be Zn—Mnalloy as a result of the EPMA (Electron Probe X-ray Micro Analyzer)analysis, and the thickness was about 50 mm.

The hardness distribution is shown in FIG. 15. According to FIG. 15, itis clear that the hardness of the surface layer of the base material 10was remarkably improved. Further, the boundary portion was scarcelyrecognized in the surface layer of the base material 10. It wasconfirmed that the oxide film was effectively removed and the alloyformation was advanced.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method of producing a metal material comprising a diffusion layerwhich is formed by diffusing an element into a base material of a metaland which has a depth from a surface of said base material of not lessthan 0.5 mm, a concentration of said element being gradually decreasedfrom said surface to inside of said base material, said methodcomprising: coating said surface of said base material with a coatingagent, said coating agent consisting essentially of a powder of asubstance containing said element to be diffused, a reducing agent forreducing an oxide film formed on said surface of said base material, anda solvent in which said powder of said substance and said reducing agentare dispersed or dissolved; and diffusing said element into said basematerial by heating said base material which is coated with saidsubstance such that the element is diffused into the base material to adepth of not less than 0.5 mm from the surface, wherein said diffusingstep causes the concentration of said element to be gradually decreasedfrom said surface to inside of said base material such that an interfaceis not formed between the base metal material and the diffusion layer,and the surface of the base metal material having the element diffusedtherein is exposed outwardly after said diffusion step.
 2. The method ofproducing said metal material according to claim 1, wherein said basematerial comprises a Zn alloy, and wherein a resin of at least one ofnitrocellulose, polyvinyl alcohol, polyvinyl, acrylic, melamine,styrene, and phenol is used as said reducing agent.
 3. The method ofproducing said metal material according to claim 2, wherein said coatingagent further contains at least one metal powder of magnesium, aluminum,or manganese, or at least one alloy powder of a magnesium alloy, analuminum alloy, and a manganese alloy.
 4. The method of producing saidmetal material according to claim 1, wherein said base material isheated such that a temperature gradient is formed in said diffusingstep.
 5. The method of producing said metal material according to claim1, wherein said diffusing step is carried out in an inert gasatmosphere.
 6. The method of producing said metal material according toclaim 3, wherein at least one of Ni, Sn, and Cu, is further added tosaid coating agent.
 7. The method of producing said metal materialaccording to claim 1, wherein the reducing agent is decomposed by theheating in said diffusing step, and a product of decomposition causesthe oxide film on the surface of the base material to disappear byreduction so that the element is diffused into the base material.
 8. Amethod of producing a metal material comprising a diffusion layer whichis formed by diffusing an element into a base material of a metal andwhich has a depth from a surface of said base material of not less than0.5 mm, a concentration of said element being gradually decreased fromsaid surface to inside of said base material, said method comprising:coating said surface of said base material with a coating agent, saidcoating agent including a powder of a substance containing said elementto be diffused, said coating agent further including a reducing agentfor reducing an oxide film formed on said surface of said base material,and said powder of said substance and said reducing agent beingdispersed or dissolved in a solvent; and diffusing said element intosaid base material by heating said base material which is coated withsaid substance, wherein a Zn alloy is used as said base material, atleast a part of said base material is coated with a first powdercontaining at least any one of copper and manganese, and then the basematerial having said powder with at least one of copper and manganesecoated thereon is coated with a second powder containing Fe in saidcoating step.